1. Field of the Invention
The present invention relates to molded articles having the appearance of natural cork, such as closures for liquid product containers, and to methods and apparatus for making the same. More specifically, the present invention relates to the use of a particular colorant in the molding of synthetic plastic closures which gives the closures the appearance of natural cork.
2. Description of Prior Art
It is known heretofore to manufacture articles from foamed thermoplastic materials by injection or extrusion of polyurethane, polystyrene or polyolefins. In these cases, the thermoplastic material may be expanded from a granular form to assume a porous, solidified foam-like structure by the action of various propellants or agents for expanding or blowing the materials. The blowing agents, in accordance with normal practice, are usually gases or gas generating substances that have been dissolved or otherwise intimately incorporated within the thermoplastic resinous material while it is in an unexpanded granular form. The application of heat to such a mixture causes the blowing agent to be released or thermally expanded or both, while the thermoplastic material is attaining a foaming temperature at which it is sufficiently softened and yieldable to permit the pressure of the expanding blowing agent to expand it into the desired foam structure.
In a special case of extrusion forming of foamed thermoplastic materials, it is also known to decorate the surface of articles so produced to resemble natural wood by imparting to the surface a parallel grain structure. Such decoration is accomplished by extruding the material through a die having, in particular, a multiple number of apertures to form a corresponding number of strands of expanding resin, collecting these strands in parallel relationship, and then coalescing them into a body of any desired length. Contacting surfaces of the strands create parallel grain lines which remain clearly visible in the surface of the finished articles and resemble natural wood grain.
In a special case of injection molding of foam thermoplastic materials, it is known to mold articles by injecting into an enclosed cavity a measured amount of expandable thermoplastic resin so that the cavity is not completely filled by the unexpanded material but becomes filled as the material expands. As expansion occurs, the softened resin is forced by a gaseous blowing agent into contact with the relatively cool walls of the mold cavity where the portion of the resin in direct contact with the walls is solidified and forms a dense outer layer or skin conforming to the shape of the cavity and functioning as a barrier, thereby preventing the escape of the gaseous blowing agent from the article and causing gas bubbles to be entrapped in the article's interior region. Thus, articles molded in this manner are characterized by a porous, foam-like interior and a dense outer shell or skin having the same general size and shape as the mold cavity and which are essentially impermeable to most substances.
By employing certain highly flexible thermoplastic materials, for example polyolefin copolymers, such as ethylene vinyl acetate, in the previously described injection molding process, the resulting foamed articles will exhibit elastic properties closely resembling similar products manufactured from other natural and synthetic elastomers. This flexibility and resiliency plus the presence of an essentially impermeable skin allows these same articles, if molded in the proper size and shape, to function as closures for liquid product containers by being compressed and fitted into the container opening, then being allowed to expand, filling the opening and creating a continuous seal about its perimeter. Therefore, such articles would theoretically be useful as replacements for similar closure devices manufactured of more familiar materials, such as natural cork used in containers of medicine, wine and spirits.
In reality, efforts at replacing natural cork closures in liquid product containers, especially containers for wine products, by manufacturing and utilizing closures from injection molded foamed thermoplastics have not been very successful. This has been due primarily, it is believed, to the lack of asethetically created consumer appeal for such closures, and the relationship of the dense outer skin to the porous inner core which dense outer skin, while necessary to make the closure essentially impermeable, can be a detriment to the sealing function of these articles, particularly when leakage channels are formed on the dense outer skin during the sealing of these articles.
Since such prior art closures must be compressed to be fitted into liquid product containers there is a tendency for the dense outer layer of the closures to wrinkle and form longitudinal channels in the outer surface of the foamed articles. Such channels could form leakage paths for the container contents or allow the ingress of substances harmful to the product being contained, such as ambient air in the case of the medicine or food products. Tests have shown that these channels are caused primarily by the presence of relatively large void spaces between the dense outer skins and the porous inner core of the foamed articles, their presence being the result of a previously unrecognized phenomena occurring during the molding of the articles. Since the dense outer skin has a much higher resistance to compression than the porous inner core, it tends to deflect into any available void space, creating a channel on the outer surface of the closure that could result in a disruption of the otherwise continuous perimeter seal.
In addition to the foregoing problems, prior art molded closures have not been uniform and consistent in design, color and performance. Thus, such prior art closures have not been found readily acceptable as a replacement for similar closure devices, such as natural cork used in containers of medicine, wine and spirits.
In application Ser. No. 202,156 a new and improved method and resulting product were developed for making a closure made of thermoplastic resinous material having a natural, cork-like appearance, which substantially eliminated the prior art problems discussed hereinabove. According to the invention disclosed in this application a thermoplastic resin is injected into one or more mold cavities through a single orifice in each mold cavity which forms the material into a single and continuous elongated strand within each mold cavity. As the strand length builds in each mold cavity, it is contained by the cavity walls which force the single and continuous elongated strand to collect in a random fashion when the strand meets resistance within the mold cavity and contacts itself at various points along its length. The expansion of the blowing agent causes the randomly positioned loops of the strand to expand and coalesce leaving grain lines in the surface of the molded article similar to those found in the surface of extruded articles manufactured in accordance with previously known teachings but which, in distinction, are not straight or parallel but of varying direction, occasionally intersecting, and which are formed from a single and continuous elongated strand of resin. Also, these articles contain the randomly oriented grain lines on the entire outer surface, including the ends thereof, and thus constitutes an improvement over articles manufactured by extrusion that normally do not contain randomly oriented grain lines on the ends of the articles. These randomly oriented grain lines, when embodied in the entire surface of an article having the same general size and shape as the natural cork portion of a closure for a liquid product container, cause the article to resemble natural cork in appearance. Such an effect is achieved by avoiding an abrupt expansion of the thermoplastic material as it is injected into the mold cavity. Thus, by retarding the expansion of the thermoplastic material sufficiently to allow completion of the injection of said thermoplastic material into the mold cavity, the integrity of the strand configuration is maintained, which, in turn, is reflected in the formation of grain lines on the surface of the closure which is thus produced. The retarding of the expansion of the thermoplastic material is achieved through temporary pressurization of the mold cavity which is achieved by rapidly injecting at high pressure, an expandable thermoplastic resinous material into said mold cavity and gradually removing a portion of the air displaced by said injected thermoplastic resinous material through controlled minimum venting of the mold cavity and expanding the coiled resinous material in a controlled manner through a continuation of the minimum venting to fill the mold cavity.
Resistance to wrinkling during subsequent compression of the article is improved to a satisfactory level by incorporating into the molding method a specific technique and series of embodiments to the molding equipment which, together, eliminate the tendency of the foaming material to form large void spaces between the dense outer skin and the foamed interior of the article. This is accomplished by insuring that all the resin material to be utilized in each article enters the mold cavity, and that each cavity entrance is positively sealed prior to the expansion of the resin, to form a solidified outer skin adjacent to the cavity wall.
It was found in prior molding techniques that further ingress of material after the relatively rigid and dense outer skin was formed, sets up a shear boundary layer within the material being molded and results in the still soft inner core of the article shifting relative to the dense outer skin, thus breaking the material bond between the dense outer skin and the inner core. Gas pressure subsequently forces the porous inner core of the article away from the dense outer skin thereby creating large void spaces immediately beneath the rigid and dense outer skin. The lack of a complete bond between the dense outer skin and the porous inner core results in, during compression of the molded article, relative movement between the two components (i.e. dense outer skin and porous inner core) of the molded closure device, which results in wrinkles and leakage paths on the outer surface of the molded closure device.
Chronological separation of injection and expansion steps is achieved by several embodiments to the improved molding technique and equipment, including:
1. The maximization of the available injection pressure and speed with a corresponding minimization of venting within each mold cavity of a plurality of mold cavities with the result, during injection, of a temporary pressurization of each cavity caused by rapid ingress of expandable resin and the opposing slow escape of displaced gas which restricts the expansion of the thermoplastic resin to form a solidified exterior until the injection of each cavity is completed.
2. The creation of a runnerless molding technique by the incorporation within the single small orifice of each mold cavity through which all material must flow to reach each mold cavity of a positive means of shutting off the flow of material to or from each cavity such that no continued channel for resin flow exists between each shutoff in its associated cavity and the heated runner channel following completion of the injection portion of the molding cycle of that particular cavity.
3. The utilization of each shutoff in its associated cavity as a means of initiating and terminating the injection of material into each cavity such that the period for transfer of material to or from each mold cavity through its respective injection orifice can be positively controlled.
Thus the method for making a closure made of a thermoplastic resinous material and having a natural cork-like appearance as defined by application Ser. No. 202,156 can be summarized by the steps of
providing a mold cavity having the shape of a desired closure with predetermined internal dimensions,
rapidly injecting, at high pressure an expandable thermoplastic resinous material in an amount less than that required to fill the mold cavity and in the form of a strand, into the mold cavity, the strand collecting in said cavity as randomly-oriented coils, the pressure produced in the mold cavity by the injection temporarily pressurizing the cavity, thereby retarding the expansion of the thermoplastic resinous material sufficiently to allow completion of the injection of the thermoplastic resinous material, while maintaining the integrity of the strand configuration,
simultaneously with the temporary pressurization, gradually removing a portion of the air displaced by the injected thermoplastic resinous material through controlled minimum venting of the mold cavity, thereby expanding the coiled resinous material in a controlled manner through a continuation of said minimum venting to fill the mold cavity, the walls of the mold cavity being maintained at a temperature sufficiently cool to cause the resinous material to solidify into a rigid, outer skin when it contacts the walls, whereby the coalesence of the coils during expansion creates randomly-oriented grain lines in the surface of the closure which resembles the appearance of natural cork.
Advantageously, the entrance orifice is immediately sealed after the thermoplastic resinous material has been injected into the cavity to stop any further flow of the resinous material, with the cavity and also the entrance orifice being kept in a sealed condition until the resinous material subsequently expands and solidifies to form the closure. The closure thus formed has a liquid impervious outer skin and a foamed inner core with no appreciable void spaces or separations between the outer skin and the foamed inner core into which said outer skin could collapse when compressed.
In application Ser. No. 202,156 a blend of pigments (colorants) were utilized to impart a cork-like appearance to the products. For example, in one embodiment at least 5 percent of the pigment utilized is noted as being a blend of FD&C certified alumina lake powders having a particle size of 40 to 60 microns. In another embodiment the total pigment blend was said to comprise approximately 0.5 percent by weight of the raw material formulation used in the process. However, it has been found that such colorants do not possess universal governmental acceptability and are also a relatively expensive portion of the final product.
Accordingly an object of the present invention is to provide a molded article having a cork-like appearance. A further object of the present invention is to provide a synthetic cork closure product having an improved cork-like appearance.
Another object of the present invention is to provide a new and improved colorant for a synthetic cork closure product which imparts a color to the closure which very closely resembles the color of natural cork closures.
A further object of the present invention is to provide a new and improved colorant for a synthetic cork closure product which is particularly compatible in the injection molding method for making closures as described hereinbelow.
Still another object of the present invention is to provide a new and improved colorant for a synthetic cork closure product which possesses universal governmental acceptability.
A still further object of the present invention is to provide a new and improved colorant for a synthetic cork closure product which is relatively inexpensive when compared to other colorants normally used in plastic materials.
Yet another object of the present invention is to provide an injection molding method for making a closure made of a resinous material containing a new and improved colorant which gives the closure a cork-like appearance.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
Pursuant to the present invention, plastic articles having a cork-like appearance, particularly synthetic cork closure products having a natural cork-like appearance can be obtained by incorporating into the thermoplastic material used to make the synthetic cork closure an effective amount of caramel as the colorant therefor. The use of either liquid or powdered caramel in the synthetic cork closure is effective in imparting a color which very closely resembles the color of natural cork closures. Because caramel colorant is already used in various alcoholic beverages and because it is readily recognized as a commonly used food additive, its presence as a colorant in a synthetic cork possesses universal governmental acceptability as a truly international product. Also, compared to other FDA-approved colorants normally used in plastic materials, caramel is relatively inexpensive, representing approximately 4 percent of the cost of presently known and used FD&C colorants.